This invention relates generally to a control system for accurately, and continuously controlling the weight per unit length (weight/unit length) of an uncut extrudate of arbitrary length and cross-section for which the dimensions are left uncontrolled.
More specifically this invention relates to a process and apparatus for automatically maintaining the desired weight/unit length of a continuously extruded rubber tread produced from one or more extruders, without measuring or directly controlling the area of the tread, or its transverse cross-sectional area. Plural extruders are used in tandem to produce a joined extrudate, or, to produce extrudates which are combined in a common head. This invention may equally be applied to other extrudates of elastomeric stock if one recognizes that the specific weight can be maintained though the dimensions of the extrudate may vary with changes of density.
Heretofore, the elastomeric stock used for tire treads has been extruded in a continuous strip of desired width and thickness, cooled, and cut to a required length dictated by the circumference of a green tire carcass to which it is to be applied. Variations within a production run are caused due to (i) changing rubber properties and (ii) operating conditions in the overall system, particularly variations in the operation of the extruder. In the past, the thickness of tread stock has therefore been controlled by monitoring the dimensions of the extruded tread, and also its weight, and making the necessary adjustments, whether manually or automatically, in response to the changes measured.
We have successfully been able to control the extruded tread within the strictest specifications by measuring only weight/unit length, and ignoring dimensional changes which may occur independent of weight changes. Because of its geometry, tread weight is directly related both to its thickness, and to its area. Changes in dimensions without a corresponding change in weight occur due to variation in density resulting from changes in the amount of entrained air. Therefore a dimensional change may not indicate a change in mass. By controlling only weight/unit length, we have indirectly taken advantage of this principle to simplify the task of producing "on-spec" tread stock.
Monitoring of an extrudate in the prior art is accomplished by comparing the weight/unit area of a running length of continuous extrudate as it passes over a running-weigh-scale (hereinafter referred to as he "RWS") to that of a cut length of the material taken at a checkweigh scale ("CWS") after the extrudate is cooled and skived (cut to length). This requires measuring both length and width to determine the area. Since the length is typically kept constant, based on a running unit length of extrudate over a scale, it is evident that the dimension to be monitored is width.
In the specific instance of producing tread stock, the series of equipment required to produce it, namely the extruder, takeaway conveyor, shrink rolls, auxiliary conveyors, cooling train, and skiver, along with various monitoring equipment, is referred to as the "tread line". The tread stock specification, in the prior art, is established at the CWS with the RWS providing a means for making a first approximation inasmuch as the RWS and CWS are generally at separate locations in the tread line. Once the relationship between RWS and CWS is established, the desired value of CWS can be translated into a desired value of RWS. We establish a target weight/unit length based on an ideal tread without regard for the value obtained at the CWS. By "ideal tread" we refer to a predetermined weight of tread for a particular tire. This target weight at the RWS is referred to simply as "target weight" for brevity. In our process, the weight at the CWS is checked only to determine whether the tread is on-spec.
In U.S. Pat. No. 4,233,255 to Moon, there is disclosed a two-conveyor system for controlling the weight per unit area (weight/area) of a continuous extrudate after it leaves the second conveyor. His correct, geometrically accurate analysis utilizes the principle that weight/unit area is the key to providing the correct gauge (thickness). It is implicit that he must measure both length and width to know the area, and the width is measured manually either in line, or, after the tread is cut to length. He provides no disclosure as to how he measures width automatically, either before or after the tread is cut to length.
Therefore the '255 method comprises controlling either the thickness or the weight/unit area of continuously extruded extrudate. He failed, as did others in the prior art, to recognize that one can ignore variations in both width and thickness, if the weight/unit length is controlled closely enough. Thus, in the '255 tread line, Moon uses a first variable speed take-away conveyor belt to convey the extrudate as it exits the extruder, a second variable speed conveyor belt for varying the thickness or weight of the extrudate after leaving the first conveyor belt, a RWS to measure the weight/unit area of the extrudate after it leaves the second conveyor, and a target value. His method comprises measuring the weight value of the extrudate at the RWS; comparing the RWS measured value to the target value to obtain a control signal; and adjusting the difference in speed between the first conveyor belt and the second conveyor belt in response to the control signal. This method relies on adjusting the relative speed of the extrudate as it travels between, and over, two variable-speed conveyors.
The Moon patent requires that there be an adjustable difference in the speed between the two variable-speed conveyors to control weight which he states gives the desired control, without the "thinning" or "thickening" effect obtained by controlling the extrudate with a single variable speed conveyor, as described in U.S. Pat. No. 4,088,721 to Apicella, Jr. We revert to the use of a single variable-speed conveyor and have discovered that despite the thinning and thickening effect that we experience, it does not interfere with precise control of the tread specifications. Moon sought to avoid this effect by controlling weight/unit area, thus maintaining constant thickness. Neither Moon nor Apicella appreciated how unexpectedly simple and effective the spec-control process would be if they relied solely on controlling the weight/unit length of the tread.
Referring again to the method for the control of extruded stock in the '721 patent to Apicella, Jr., note that he controls the thickness of the stock. In that patent, the RWS is continuously compared with the CWS to obtain a targeted average. The measured RWS is compared to the targeted average to obtain a control signal. The control signal is used to increase or decrease the speed of a take-away conveyor belt, positioned at the exit orifice of the extruder. If, for example, the measured RWS is too high compared to the targeted RWS, the speed of the take-away conveyor belt is increased to stretch the extrudate. This will result in a thinner tread stock. Alternatively, if the RWS value is too low, resulting in a low value of CWS, indicative of a thinner tread stock, the take-away belt may be slowed down with the result being a thicker extrusion.
In the '255 method, the takeaway conveyor at the extruder and the RWS conveyor are the two conveyors which have relatively variable speeds, providing a speed differential which dictates the weight/area, and the thickness of the extrudate. Between these two conveyors are a series of rollers to allow the tread to cool and shrink. It is only after the extrudate leaves the second conveyor that its weight is checked at the RWS. This is done deliberately to accomodate tread shrinkage, so that the second conveyor controls both the thickness and the weight. The actual degree of tread shrinkage between these two conveyors is dependent on the temperature, compound rheological properties, tension at the die opening, and speed differential between the two conveyors. These transient conditions make it difficult, accurately to adjust tread weight by adjusting the difference in speeds between the two conveyors in this region of the tread line, particularly because the transient conditions may vary quite often within a given period (say, a shift), and often from one hour to the next. Therefore, in our invention, as in the prior art, we use a shrink roll conveyor to allow for shrinkage, but because we seek to control mass, not dimension, in the instance where the shrink rolls precede our RWS (as shown in FIG. 1), we accept whatever shrinkage the shrink rolls provide.
The control of size deviation either by manipulating the output rate of an extruder, or, by manipulating the speed of a takeaway means, has been well known in the art. In cold-feed extruders, it is known to manipulate the speed of the screw to vary the extruder output and thus control size deviations. Likewise, in hot-feed extruders, it is known to manipulate the feed rate by varying the width of the strips of rubber fed into the extruder from the breakdown mill. In addition to the Moon, and Apicella patents referred to hereinabove, examples of processes in which the take-away speed is manipulated to control size deviations are also found in U.S. Pat. No. 4,087,499, issued to Bayonnet; and in U.S. Pat. No. 4,097,566 to Bertin et al. Another example of process control in which a double roller die is used to control size is shown in U.S. Pat. No. 3,975,126, issued to Wireman et al.
Our system adjusts the takeaway conveyor to maintain a constant weight/unit length as soon as the extrudate leaves the die opening in the extruduer, or as soon thereafter as is practicable if an existing tread line (referring to all the equipment currently used in the processing of tread extrudate) is to be used. This control of weight in close proximity, if not adjacent, to the die opening has the incidental effect of changing the tension on the extrudate at the die opening, which also helps produce the precise required weight at the RWS. Only the speed of a single variable-speed conveyor is used for control, and the speed of all other conveyors is adjusted with loop controls to minimize subsequent tension on the extrudate. Of course, the extrudate being extruded from the die opening is affected by some of the same parameters mentioned previously, i.e. compound temperature and rheological properties, but our system reacts and adjusts for such changes by controlling extrudate weight/unit length. Width and cross-sectional area changes are ignored because these may be affected by density changes within limits are tolerable.
This control system is easily distinguishable over one using plural variable-speed conveyors and where a second conveyor is required to maintain a target weight by stretching or shrinking the extrudate. The prior art method is less effective particularly when the second conveyor is in a portion of the tread line, after the shrink rolls, because it is known that the dimensions of the tread can be more variable after shrinkage than they are next to the extruder's die opening. Therefore in our most preferred embodiment, the RWS and takeaway conveyor are positioned adjacent the extruder.
In our system, it is also possible to modify an existing tread line where the RWS is positioned after the shrinkage conveyor, by equipping the tread line with the control means specified herein. Because of the relative distance between the RWS and the extruder in such a system, one obtains long-term control at the expense of short-term control of precise tread specifications. Stated differently, the more quickly one controls weight/unit length as it leaves the extruder, either with a take-away conveyor associated with the RWS, or with a RWS positioned following the takeaway conveyor, the better the control, and the more rapid the detection of a change in weight near the die opening.